a) Field of the Invention
The invention is directed to a method for the energy stabilization of a gas discharge-pumped radiation source that is operated in defined pulse sequences, particularly for suppression of overshoot and undershoot in burst operation of excimer lasers and EUV radiation sources.
b) Description of the Related Art
In addition to special lamps, mainly narrowband excimer lasers with wavelengths of 248 nm and 193 nm are currently used as radiation sources in photolithography processes for the fabrication of microchips. Methods based on F2 lasers (157 nm) are in development. EUV lithography (13.5 nm) appears to be the most promising variant for next-generation lithography.
In all of these lithography processes, a mask (with the structure to be imaged) is imaged on the wafer in a scanner in a reduced manner.
The usual exposure principle in the scanners mentioned above is a burst regime, as it is called, in which the wafer is not continuously exposed (because of the special steps for manufacturing a microchip), but rather the radiation source supplies sequentially defined sequences of radiation pulses. A pulse sequence (burst) of this kind contains 100 to 400 light pulses. After every burst, there is a pause during which, e.g., the work gas can recover. After this burst pause, the radiation source supplies pulse energies at a fixed operating voltage in the following burst for the first 10 . . . 40 pulses, which pulse energies are higher than for the rest of the light pulses. This behavior is known as overshoot. Depending on the gas state, the radiation source reaches a stationary state after 10 to 40 pulses. Under certain conditions, an undershoot behavior can also be observed. This occurs when the excitation conditions for the emission of the light source worsen during the burst pause. The dose fluctuations which accordingly occur impair the photolithographic process and are therefore undesirable.
The method described above has also been observed in narrowband excimer lasers. Because of the altered conditions in the burst pause, the first pulses in the burst are subject to deviations from the reference wavelength, a wavelength shift, as it is called. In this case, the aim is to reduce this wavelength shift (overshoot/undershoot shift) to below a determined acceptable value.
A regulation of energy of the kind mentioned above for narrowband excimer lasers is described in U.S. Pat. No. 6,005,879 as follows. For the first 10 . . . 40 pulses in the burst, a learning table for the charging voltage U is stored in the control computer. This learning table results from the behavior of the first pulses of the preceding bursts and is continuously updated based on the current burst. The voltage values stored in the learning table are taken into account in a modified PI regulation.
A similar procedure is followed for wavelength stabilization. In this case, the wavelength-controlling optical component (e.g., the grating of the narrowband unit) is rotated during the burst pause until the wavelength shift is reduced.
In all of the known regulations, the radiation sources are regulated differently during the start phase of the burst than during the subsequent stationary phase. Another disadvantage of the known algorithm consists in that coefficients of the PID regulation have fixed values which are determined empirically at the start. However, the pulse statistics change over the gas lifetime of the excimer laser, so that these factors must be repeatedly optimized by trial and error. This means extensive on-site measurements of the equipment by service engineers. Further, it is not easy to compensate for the starting behavior at the beginning of every burst when constant discharge conditions cannot be taken as a basis.